Also add some b.ReportAllocs calls to other image codec benchmarks.

Change-Id: I0f055dc76bffb66329c621a5f1ccd239f0cdd30b
Reviewed-on: https://go-review.googlesource.com/68390Reviewed-by: Jed Denlea <jed@fastly.com>
Reviewed-by: Nigel Tao <nigeltao@golang.org>